Inkjet printing is typically done by either drop-on-demand or continuous inkjet printing. In drop-on-demand inkjet printing ink drops are ejected onto a recording medium using a drop ejector including a pressurization actuator (thermal or piezoelectric, for example). Selective activation of the actuator causes the formation and ejection of a flying ink drop that crosses the space between the printhead and the recording medium and strikes the recording medium. The formation of printed images is achieved by controlling the individual formation of ink drops, as is required to create the desired image.
Motion of the recording medium relative to the printhead during drop ejection can consist of keeping the printhead stationary and advancing the recording medium past the printhead while the drops are ejected, or alternatively keeping the recording medium stationary and moving the printhead. The former architecture is appropriate if the drop ejector array on the printhead can address the entire region of interest across the width of the recording medium. Such printheads are sometimes called pagewidth printheads. A second type of printer architecture is the carriage printer, where the printhead drop ejector array is somewhat smaller than the extent of the region of interest for printing on the recording medium and the printhead is mounted on a carriage. In a carriage printer, the recording medium is advanced a given distance along a medium advance direction and then stopped. While the recording medium is stopped, the printhead carriage is moved in a carriage scan direction that is substantially perpendicular to the medium advance direction as the drops are ejected from the nozzles. After the carriage-mounted printhead has printed a swath of the image while traversing the print medium, the recording medium is advanced; the carriage direction of motion is reversed; and the image is formed swath by swath.
A drop ejector in a drop-on-demand inkjet printhead includes a pressure chamber having an ink inlet for providing ink to the pressure chamber, and a nozzle for jetting drops out of the chamber. Two side-by-side drop ejectors are shown in prior art FIG. 1 (adapted from U.S. Pat. No. 7,163,278) as an example of a conventional thermal inkjet drop-on-demand drop ejector configuration. Partition walls 20 are formed on a base plate 10 and define pressure chambers 22. A nozzle plate 30 is formed on the partition walls 20 and includes nozzles 32 (also called orifices herein), each nozzle 32 being disposed over a corresponding pressure chamber 22. The exterior surface of a nozzle plate 30 is called a nozzle face 114 herein. Ink enters pressure chambers 22 by first going through an opening in base plate 10, or around an edge of base plate 10, and then through ink inlets 24, as indicated by the arrows in FIG. 1. A heating element 35, which functions as the actuator, is formed on the surface of the base plate 10 within each pressure chamber 22. Heating element 35 is configured to selectively pressurize the pressure chamber 22 by rapid boiling of a portion of the ink in order to eject drops of ink through the nozzle 32 when an energizing pulse of appropriate amplitude and duration is provided.
Developments within the inkjet printing industry have increased the importance of wide printhead assemblies where the drop ejector array on the printhead can address the entire region of interest across the width of the recording medium. Although carriage printers are suitable for home and small office use, higher speed printers using pagewidth printheads are more suitable for networked printers for larger offices. A second development within the inkjet printing industry is the increased use of commercial printing. Commercial inkjet printers are capable of printing high volumes of pages at high printing throughput. A third development is the use of industrial inkjet printers for textile printing, decorative printing, graphic arts and 3D printing. Such printing systems can require print areas that are greater than one meter in width. Further printing applications that can benefit from wide printhead assemblies include deposition of biological materials, as well as functional printing of electronic circuitry.
Drop ejector arrays are typically formed using fabrication technologies developed for micro-electro-mechanical systems (MEMS) and integrated circuits. The present largest size of commercially available silicon wafers is about 30 centimeters in diameter. Although it would be possible to make pagewidth printheads having a width less than 30 centimeters using a single printhead die from such a wafer, manufacturing yield is such that it is economically advantageous to assemble a pagewidth printhead using printhead dies that are on the order of 1 centimeter wide. The drop ejector arrays on each of the printhead dies need to be well-aligned with each other. Otherwise there will be unacceptable defects in printed images, such as white streaks resulting from endmost drop ejectors on two adjacent printhead dies being too far apart from one another.
Two generic configurations of printhead assemblies are those that use overlapping printhead dies and those that use butted printhead dies. In an assembly of overlapping printhead dies each printhead die is longer than Nd, where N is the number of drop ejectors in the array on a single printhead die, and d is the distance along the array direction between adjacent drop ejectors. As a result, such printhead assemblies cannot have adjacent printhead dies arranged end-to-end because an unacceptable gap would result between endmost drop ejectors on adjacent printhead dies. A variety of ways have been disclosed for accommodating the printhead die length in an assembly of overlapping printhead dies while still providing an arrangement of drop ejectors that can print acceptable images.
U.S. Pat. No. 4,520,373 discloses a pagewidth printhead including overlapping printhead dies that are alternately adhered on both sides of a metal heat sink. This configuration is compatible with drop ejector geometries where the nozzles are formed in an edge of the device. U.S. Pat. No. 4,559,543 discloses a similar configuration where each printhead unit is detachably mounted in staggered fashion on opposite sides of a support bar so that damaged printhead units can be replaced. Complex adjustment capability is built into the print bar for aligning the printhead units. U.S. Pat. No. 5,257,043 discloses a similar configuration where modular printhead units are arranged in staggered fashion on opposite faces of a support bar. The printhead units are releasably positioned on the support bar by mechanical contact of the printhead against either external jigging or patterned features that are permanently fabricated on the support bar faces.
For drop ejector geometries where the nozzles are formed in a face of the device, the printhead dies can be aligned in multiple rows on a single surface of a carrier substrate. Such an arrangement is disclosed in U.S. Pat. No. 6,250,738 where a scalable printhead is formed by mounting an ink manifold and multiple thermal inkjet printhead dies to a carrier substrate. The carrier substrate is machined to include through-slots for providing ink passageways between the ink manifold and each printhead die. Alignment of the printhead dies is accomplished by solder reflow forces that cause precisely located wetting metal patterns on the printhead dies to line up with corresponding precisely located wetting metal patterns on the carrier substrate, as disclosed in U.S. Pat. No. 6,123,410.
U.S. Pat. No. 7,384,127 discloses an alternative alignment approach for staggered rows of printhead dies. Each printhead die is affixed within a recess of a corresponding precision micro-molded printhead segment carrier. The printhead segment carriers have stepped ends for nesting in alternating orientation to provide an overlapping staggered arrangement of printhead dies. Lengthwise alignment between successive printhead segment carriers is accomplished by positioning the carriers using fiducial marks on the front surface of each of the printhead dies. The carriers are then bonded in position along a support.
A different configuration for accommodating overlapping printhead dies is to position each printhead die at an angle with respect to a straight line running the length of the printing zone, thereby enabling overlap of the ends of adjacent printhead dies, as disclosed in U.S. Pat. No. 6,994,420. The printhead dies are positioned in carriers and include fiducials in the form of markers to facilitate accurate alignment. U.S. Pat. No. 7,152,945 discloses that firing of the diagonally overlapping printhead dies can be adjusted digitally during printing rather than relying on very close tolerances for alignment.
For printhead dies having a length that is substantially equal to Nd, the printhead dies can be butted end to end without an unacceptable gap between endmost drop ejectors of adjacent printhead dies. Various alignment schemes have been disclosed for printhead assemblies using butted printhead dies. The drop ejectors are arranged along a single direction rather than being overlapping, offset and staggered. Arrangement of the drop ejectors along a single direction is preferable for facilitating precision alignment, for compactness of the wide printhead assembly, and for ease of image processing.
U.S. Pat. No. 4,690,391 discloses a method and apparatus where each buttable die is provided with a pair of V-shaped locating grooves in its face. An aligning tool has pin-like projections that are insertable into the locating grooves, so that the aligning tool is used to position a series of the dies in end-to-end fashion. Vacuum ports in the aligning tool draw the dies into tight face-to-face contact with the tool. A suitable base is then affixed to the aligned dies and the aligning tool is withdrawn. As pointed out in U.S. Pat. No. 4,975,143, a limitation with the aligning tool of '391 is that the accuracy of the location of the dies is a function of the accuracy with which the alignment structures can be formed on the tool. An improvement disclosed in '143 is that the alignment pattern on the alignment tool is formed in a photo-patternable or electroformable material for improved accuracy of the alignment tool.
As described above with reference to '391, in some printhead assemblies the printhead dies are all directly bonded to a common base. U.S. Pat. No. 5,079,189 discloses an alternative configuration where each die is mounted separately on a planar support to form a subunit. The width of the support is less than the width of the die, so that the side edges of the die extend outwardly beyond the side edges of the planar support. Subunits are aligned on a substrate bar by butting the extending side edges of the die in adjacent subunits, and by butting the front edges against an alignment tool.
Forming butting edges without damage and at precise locations relative to the drop ejectors is important. U.S. Pat. No. 4,822,755 discloses a method for separating dies formed on a silicon substrate using reactive ion etching techniques combined with orientation dependent etching or dicing to yield integrated circuit dies having edges that can be more precisely butted together.
Mechanical contact of plain butting edges of two adjacent printhead die can be effective in providing alignment of drop ejectors along the array direction, but it is not effective in providing alignment in a direction perpendicular to the array direction. U.S. Pat. No. 6,502,921 discloses a printhead die configuration having a protruded abutting portion and a recessed abutting portion that is shaped to engage a protruded abutting portion that is formed on another printhead die.
U.S. Pat. No. 8,118,405 discloses alignment features including one or more projections on one butting edge and corresponding indentations on the opposite butting edge of the printhead die. The projections are sized to fit into the indentations of an adjacent printhead die such that when the projections contact the indentations of the adjacent printhead die, the two printhead dies are aligned relative to one another in two dimensions. Projections and indentations can have a variety of shapes, including triangular, trapezoidal or rounded as long as the indentations of one printhead die have the proper shape and dimensions to contact the projections of the adjacent printhead die and provide relative alignment. The projections and indentations can have complementary shapes.
Because wide printhead assemblies are expensive to fabricate, it is advantageous to assemble the wide printhead using a plurality of readily replaceable printhead units. Then, if a printhead unit is damaged, the quality of the wide printhead assembly can be restored by replacing the damaged printhead unit. It is particularly advantageous if the printhead units can be field replaceable. Replacing printhead subunits in the field should not require optical alignment, external jigging or complex position adjustment to align the new printhead unit. Mechanical alignment using complementary features is well-suited to this. The alignment tolerances between adjacent printhead dies are typically less than ten microns in order to provide good image quality. Mechanical alignment features providing such tolerances with respect to the drop ejectors need to be formed directly on the printhead dies that contain the drop ejectors. Such mechanical alignment features on the printhead dies need to be small so that they will not interfere with drop ejectors, ink passageways or electronics on the printhead dies. However, such small mechanical alignment features formed on the printhead dies can be fragile.
What are needed are alignment structures and methods of assembly for forming wide printhead assemblies using a plurality of printhead units that can be readily and precisely aligned to provide drop ejectors that are arranged along a single direction. Furthermore, what are needed are structures that help to protect the complementary mechanical alignment features on the printhead dies from damage.